Influence of Y2O3 on the structure of Y2O3-doped BaTiO3 powder and ceramics

International Journal of Engineering Research & Science (IJOER)

ISSN: [2395-6992]

[Vol-4, Issue-2, February- 2018]

Influence of Y2O3 on the structure of Y2O3-doped BaTiO3 powder and ceramics Ana María Hernández-López1, Sophie Guillemet-Fritsch2, Zarel Valdez-Nava3, Juan Antonio Aguilar-Garib4, Christophe Tenailleau5, Pascal Dufour6, Jean-Jacques Demai7, Bernard Durand8 1

1,2,5,6,7,8

CICFIM, Universidad Autónoma de Nuevo León, San Nicolás de los Garza, N.L., MX 66455.

CIRIMAT, Université de Toulouse, CNRS, Université de Toulouse 3-Paul Sabatier,118 route de Narbonne, 31062, Toulouse Cedex 9, France.

3 LAPLACE, Université de Toulouse, CNRS, INPT, UPS, France.

4 Universidad Autónoma de Nuevo León, FIME, San Nicolás de los Garza, N.L., MX 66455.

Abstract—Barium titanate (BaTiO3) doped with rare-earth elements (REE) is used as dielectric in the manufacture of multilayer ceramic capacitors (MLCCs). The most common REE oxide employed as dopant for this application is Y 2O3. The behavior of the Y3+ in the BaTiO3 structure depends on its concentration and the sintering conditions, among other factors, which can induce the formation of secondary phases that are a potential cause a detriment to the electrical properties of BaTiO3. The purpose of this work is to perform a phase characterization of BaTiO 3 doped with different concentrations of Y2O3, validating its possible contribution to the formation of secondary phases. The role of Y 2O3 was evaluated on two kinds of raw materials. The first one is pure BaTiO3 (< 100 ppm Y) and the second kind is a commercial formulation designed for MLCCs known as X7R (-55°C and 125°C, 15% tolerance), which among other elements, already contained 1 wt% of Y 2O3.

High concentrations of Y2O3 (1% up to 20 wt%) were used aiming to promote structural changes, and even the formation of secondary phases in amounts suitable to be detected by X-ray diffraction. Heat treatment of powder and sintering of ceramics (powder compacted at 2 MPa) were conducted in air (1310°C in air for 3 h, two steps: 1350°C then 1150°C 15 h).

A phase transition from tetragonal to a mixture of tetragonal and cubic was observed as Y 2O3 concentration increases in the thermally treated powder and in the corresponding ceramics. Commercially formulated powder showed higher densification than pure BaTiO3, and produced cubic structure at higher Y2O3concentrations. The phase Ba6Ti17O40is detected in the 20 wt% Y2O3-doped sample.

Keywords—BaTiO3, doping, Y2O3.

I.

INTRODUCTION

BaTiO3 presents interesting electromagnetic properties and has become the main component of the formulation of the dielectric material for multilayer ceramic capacitors (MLCCs) [1, 2]. The formulation used in this application must be designed to control the electromagnetic properties of the layer, especially at high temperature and under high electric field [3,4]. For this purpose, several additives and dopants are added to BaTiO 3. They include cations such as Mn, Mg and Ca, that can partially compensate the electrons and holes that the system might contain,due to the presence of oxygen vacancies [3, 5]. They also include sintering aids, such as SiO2, which reduce the sintering temperature. Indeed, it has been reported that SiO2 leads to formation of a liquid phase from the ternary system BaO-TiO2-SiO2, diminishing the eutectic point from 1320°C to near 1260 °C [6,7]. Finally, REE are added, Dy3+, Ho3+, Sm3+, La3+, Yb3+or Y3+. They substitute Ba and Ti cations in the BaTiO3 structure [8, 9]. However, in particular, Dy3+, Ho3+ and Y3+, have shown an amphoteric behavior (occupying A- or B-site) and they are described as helpful for the lifetime of the MLCCs [1]. Y2O3 is commonly employed as dopant in the commercial formulation of powder for fabrication of MLCCs, because at industrial scale. It results in similar properties than adding Ho2O3, Er2O3 or Dy2O3, and it is less expensive[8]. Dopants also take part in the formation a so-called “coreshell” structure that is claimed to contribute to the temperature stability of the dielectric properties [8-11]. Y2O3 ionic radius (0.107 nm) is intermediate between that of the Ba 2+ ion (0.161 nm) and the Ti4+ ion (0.06 nm). Therefore Y3+can take either Ba2+ or Ti4+ cation site in the BaTiO3 lattice [1, 2], and can behave as acceptor or donor according to the position in the lattice. The energy required to form a Ti4+ vacancy in the BaTiO3 lattice is 7.56 eV whereas it is only 5.94 eV to form a Ba 2+ vacancy [12-14]. The partial pressure of oxygen and sintering temperature will also induce the formation of Ba 2+or Ti4+ vacancies, leading Y3+ to occupy either one or both of them [12, 14]. This will be influenced also by the Ba/Ti ratio, the dopant concentration and its solubility, which varies according to Y 3+ taking either the Ba- or the Ti-site. Zhi et al. [15] indicated a solubility of Y3+at the Ba-site of about 1.5 at% when sintering in air at 1440 – 1470°C, while it reaches 4 at% Page | 7

International Journal of Engineering Research & Science (IJOER)

ISSN: [2395-6992]

[Vol-4, Issue-2, February- 2018]

when sintering under reducing conditions [16]. For the Ti-sites instead, the solubility is higher ≈12.2 at% at 1515°C when sintering in air.Wang et al. [2] reported that the introduction of Y3+in the BaTiO3 lattice, can lead to structural changes, as phase transformation from tetragonal to cubic. Also, it has been observed that the solid solubility of the dopant in the BaTiO3 is surpassed, when secondary phases are formed as precipitates. In the case of Y 3+, Belous et al. [17]reported the formation of the Ba6Ti17O40 and Y2Ti2O7 as secondary phases. The pyrochlore phase Y2Ti2O7 was evidenced byYoon et al. [3] and Zhang et al. [8] and they suspected that these phases are detrimental to the reliability of BaTiO 3-based MLCCs. In most industrial processes the dopant level is below 1 wt% [12] and the sintering is performed in a reducing atmosphere.

Whereas in this research, doping levels are much higher and sintering is carried out in air. To understand the role of Y 3+ addition in the formation of second phases a large range of dopants levels need to be explored up to and above the solubility limit. In this work we aim to investigate the structural changes of BaTiO3 structure when adding Y2O3, induced by high doping levels (>1 wt%) when sintering is performed under air.

II.

MATERIALS AND METHODS

Powder of BaTiO3 doped with Y2O3 was prepared (according to the formula Ba1-xYxTi1-x/4O3) by traditional solid state reaction. Two types of BaTiO3were used, “BT-A” with commercial reagent-grade purity (Ferro Electronic Materials Inc.), and “BT-B” with formulated composition for its application in MLCCs production, containing additives, mainly: Y 2O3: 1.05%; SiO2: 0.30%, CaO: 1.34%). Each BaTiO3 powder was mixed with the necessary amount of Y2O3 to obtain doped samples at 2.5, 5 and 20 wt%, then they were ball-milled in a polyurethane mill bottle with yttrium-stabilized zirconia balls, using ethanol as the grinding media for 4h.

With the aim to observe any structural change induced by doping, the powder (x = 2.5 to 20 wt%) was thermally treated at 1310°C in air atmosphere for 3h. Ceramics were prepared with the powder containing 2.5 and 5 wt% Y2O3. Then the powder was dried, 1 wt% polyvinyl butyral (PVB) was added to the powder that were mixed, grinded and sieved. Finally it was compacted using uniaxial pressing, producing discs of 8 mm in diameter. The green discs were sintered in air using a tube furnace, under a two-step sintering protocol which reached 1350°C and then remained at 1150°C for 15 h.

The crystalline phase of powder and sintered samples was characterized by X-ray diffraction (XRD, Model D4 Endeavor, Bruker AXS), using the Cu-(= 1.54 Å) radiation in the 2θ range 10 - 80°. The bulk density of the ceramics was determined from the samples mass and geometry. Powder morphology was analyzed by scanning electron microscopy (SEM, JSM-6510LV, JEOL). The particles size was determined from the micrographs using Image J software.

III.

RESULTS AND DISCUSSION

The crystal structure and the densification of the different samples are shown in Table 1.

TABLE 1 IDENTIFICATION, CHEMICAL COMPOSITION AND STRUCTURE OF THE STUDIED SAMPLES BaTiO3 raw material

Y2O3 wt%

BT-A (reagent-grade) BT-B (commercially formulated)

3.1

0 2.5 5 20 1 2.5 5 20

Crystalline structurea Powder Ceramic A0 T T A2.5 T T+C A5 T T+C A20 T+C -B1 T T+C B2.5 T+C C B5 T+C C B20 T+C -a Preponderant phase. T: Tetragonal; C: Cubic.

Sample ID

Secondary phase -------Ba6Ti17O40

Densification (%) 93.7 60.8 61.0 -96.2 80.1 72.6 --

BaTiO3Powder

SEM images of the undoped BaTiO3 powder, A0 and B1, (Fig. 1 (a and c, respectively)) show that both powder present particles with a coarse-faceted morphology. Two sizes of particles are observed, a range of particles average size of 0.48μm and 1.25 μm for the A0 and of 0.46 μm and 0.81 μm for the B1. Although the values are close the maximum average value of the B1 particles is lower than the one of the A0 particles, which is due to the thermal treatment given to the formulated powder, as requirement for its use in the MLCCs production process. On the other hand, the SEM pictures of the 5 wt% Page | 8

International Journal of Engineering Research & Science (IJOER)

ISSN: [2395-6992]

[Vol-4, Issue-2, February- 2018]

Y2O3-doped BaTiO3 powder thermally treated (Fig. 1 (b and d)), show more spherical and defined particles, compared to the undoped powder. They present a more uniform size (A5: 0.58 μm and B5: 0.53 μm) and it is possible to observe aggregates, which might evidence an incipient sintering during the applied thermal process at 1310°C.

FIG. 1. SEM images of undoped BaTiO3: (a) A0, (c) B1 and 5 wt% Y2O3 (TT): (b) A5, (d) B5 powder. (TT: Thermally treated). Scale bar: 1μm.

X-ray diffraction patterns corresponding to the thermally treated powder are presented in Fig.2. Diffraction patterns of powder obtained from the BT-A [(a) A0, A2.5, A5 and A20] fit well with the tetragonal phase. The insets in Fig. 2.a show the peaks associated to the (111) as well as (002)(200) planes of this phase. Further, on the inset of the diffraction peak at 2θ about 45°, the split in two peaks, is characteristic of a tetragonal phase [2]. For the A20 powder, the planes (002) and (200) suffer a slight distortion and the plane (111) shifts to a lower angle. Moreover for the A5 and A20 samples, the peaks corresponding to Y2O3 appear, meaning that at least a part of it remains free outside of the BaTiO 3 lattice, indicating that the solubility limit was surpassed. The shift to lower angles of some peaks is related to an expansion of the unit cell volume due to the substitution of Ba2+ by Y3+ ions.

FIG. 2. X-ray diffraction patterns of undoped BaTiO3 (1350°C) and Y2O3 - 2.5, 5 and 20 wt% - doped BaTiO3 (1350°C) powder. Powder coming from (a) BT-A and (b) BT-B.

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International Journal of Engineering Research & Science (IJOER)

ISSN: [2395-6992]

[Vol-4, Issue-2, February- 2018]

On the other hand, the XRD patterns corresponding to the BT-B powder show more changes according to the increasing Y2O3 concentration. In this case, the raw material already contains about 1 wt% of Y 2O3. The pattern of powder B1 clearly shows a tetragonal phase, whereas those of powder B2.5, B5, and B20 evidence a mixture of tetragonal and cubic phases. For the B20 powder the split of (002) and (220) is more clear than for powder B2.5 and B5 and a shift to lower angle of the (111) peak is also noticed. Contrary to the A samples, the peaks corresponding to Y 2O3 are detected only for the powder B20, implying the remaining of most of it as a free oxide. Moreover, for B20, three additional peaks appear between 2θ ≈ 29.5 30°. They do not match with the BaTiO3 nor with the Y2O3 phases. They could be attributed to the interaction of the Y2O3 excess with the BaTiO3 matrix in conjunction with that of the additives already present. Considering the possible formation of secondary phases when the solubility of Y2O3 has exceed the limit of 1.5 at% for the Ba-site and 12.3 at% for the Ti-site (in air atmosphere) [15], a screening for secondary phases was made, finding a match with Ba6Ti17O40 (JCPDS 35-0817).

BaTiO3and TiO2 are likely to form a eutectic around 1320° - 1330°C and if SiO2 is present, the eutectic point temperature decreases down to 1245° - 1260°C [18, 19]. Even without the presence of a liquid phase [20], the interaction between the dopants and the additives under our experimental conditions, could lead to a Ti-rich phase forming the secondary phase Ba6Ti17O40. This second phase has been reported previously [14, 17].

3.2

BaTiO3 Ceramics

Ceramics are prepared by sintering undoped and doped powder, BT-A (A0, A2.5, A5) and BT-B (B1, B2.5, B5), in the conditions described in the previous section.

Fig. 3 shows the X-ray diffraction patterns of the ceramics. The undoped ceramic (A0) crystallizes in a tetragonal phase, while the doped ones (A2.5 and A5) consist in a mixture of cubic and tetragonal perovskite phases as shown in the inset (Fig. 3(a)), zone around 2θ ≈ 45° where the split of the (200) peaks is deformed. The influence of the Y 2O3 over the BaTiO3 structure is greater in the case of the ceramics. Comparing the XRD pattern from the B1 powder (Fig. 2 (b)) and the corresponding ceramic (Fig. 3 (b)), it is possible to see (inset 2θ ≈ 45°) that there is no peak splitting indicating a phase transformation from tetragonal to a cubic [10-12]. The same behavior is observed for B2.5 and B5 ceramics. This behavior points out the strong interaction between additives and dopants added to the BaTiO3, which was also observed in the SEM images (Fig. 1 (d)). The way in which the addition of Y 2O3 affects the BaTiO3 structure, is also reflected in the densification of the ceramics (Table 1). Even when in both cases (BT-A and BT-B) the densification decreases as the Y2O3 concentration increases, the ceramics issued from the BT-B powder present higher densification values. This is not surprising as one of the purposes of the added compounds to the BaTiO3powder is to improve its densification.

FIG. 3. X-ray diffraction patterns of undoped BaTiO3 and Y2O3 - 2.5, 5.0 and 20 wt% - doped BaTiO3 ceramics.

Ceramics resulting from (a) BT-A and (b) BT-B.

IV.

CONCLUSION

Pure and commercially formulated BaTiO3 powder was doped with Y2O3 by traditional solid state reaction. The Y2O3 was added to pure BaTiO3 and a commercial formulation that already contained 1 wt% of Y 2O3, so that final concentrations were 2.5, 5 and 20 wt%. The samples, as powder and as ceramics, were analyzed by XRD. It was observed that as the Y 2O3 Page | 10

International Journal of Engineering Research & Science (IJOER)

ISSN: [2395-6992]

[Vol-4, Issue-2, February- 2018]

concentration increases, the effects over the BaTiO3 structure are more evident. The phase transition from tetragonal to a mixture of tetragonal and cubic was observed either in powder thermally treated as in the corresponding ceramics obtained from them. The results indicate that the additives in the commercial formulation have a strong interaction allowing a better densification and a change of phase more than in the powder and ceramics from pure BaTiO3. It was even observed the formation of a secondary phase identified as Ba6Ti17O40 in the Y2O3-doped (20 wt%) commercially formulated BaTiO3 thermally treated powder. No secondary phases formed by BaTiO 3 and Y2O3 were found, free Y2O3 is evidence of surpassing its solubility limit. The changes in the lattice show that Y entered into the lattice, and induces the formation of a secondary phase without Y or other elements of the formulation. These tests were conducted in air, so the oxides were not reduced during the treatments.

ACKNOWLEDGEMENTS The authors thank the financial support of the National Science and Technology Council of Mexico (CONACyT) and the Postgraduate Cooperation Program (PCP-RU2I), project 229286, between Mexico and France. We are also grateful with Kemet de México and Marion Technologies, France.

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